This invention relates generally to a heat exchanger for waste heat recovery from high temperature industrial exhaust streams.
The Environmental Protection Agency (EPA) estimated that 23% of the major or energy intensive industrial fuels and electricity energy consumption is discharged as waste heat in flue gases. Industry has attempted to decrease the amount of this lost energy with waste heat recovery systems (recuperators or regenerators). However, because of the high temperatures and corrosive constituents in the waste heat streams, lack of durability of the construction materials has limited recovery. Recovery of waste heat from high-temperature industrial gas streams is most commonly done by preheating combustion air using either a recuperator or a regenerator. Generally, a a recuperator uses the exhaust gas to heat the combustion air directly or through a partition wall. A regenerator normally allows hot exhaust gas and combustion air to move alternately through the same passage, thus indirectly heating the combustion air. Historically, recuperators and waste heat boilers have been constructed of metal. However, these conventional metallic heat exchangers cannot survive for extended periods in high-temperature, dirty environments without incurring severe performance penalities. Failure of the system affects not only its ability to recover waste heat, but also the operation of the process to which it is attached. Corrosion has debilitating effects on most all metals, causing premature failures and/or excessive leakages. Fouling decreases heat transfer rate, increases pressure drop, and adds expense due to increased surface area and the necessity for cleaning and refurbishing.
Ceramics, an alternative to metals, allow significantly higher material temperatures and offer resistance to many corrosive constituents in industrial waste streams. However, certain technical limitations exist which severely restrict the application of advanced ceramic heat exchangers in high temperature fouling and corrosive waste streams. These include the high costs for ceramic fabrication, problems with satisfactorily joining ceramics, the lack of data and accurate methods to predict thermal-structural behavior of ceramics especially as affected by long-term exposure to corrosive environments, the sensitivity of some of the more corrosive-resistant ceramics to thermal shock fracture, and the inability to detect and evaluate flaws causing failure.
Some major waste heat streams include glass melting, aluminum remelt, and steel soaking and reheat furnace flue gases. Although many other sources of waste heat exist, these three represent the largest quantity of waste heat. The streams are very high temperature, generally 1500.degree. to 2800.degree. F. and usually contain highly corrosive constituents which degrade the materials and contain particulates that tend to build up and foul the heat exchanger.
Therefore it is an object of the present invention to provide a ceramic heat exchanger suitable for use in waste heat streams.
It is another object of the present invention to provide a heat exchanger suitable for use at high temperatures and resistant to corrosion and fouling.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.